670 research outputs found
Photoexcitation in two-dimensional topological insulators: Generating and controlling electron wavepackets in Quantum Spin Hall systems
One of the most fascinating challenges in Physics is the realization of an
electron-based counterpart of quantum optics, which requires the capability to
generate and control single electron wave packets. The edge states of quantum
spin Hall (QSH) systems, i.e. two-dimensional (2D) topological insulators
realized in HgTe/CdTe and InAs/GaSb quantum wells, may turn the tide in the
field, as they do not require the magnetic field that limits the
implementations based on quantum Hall effect. Here we show that an electric
pulse, localized in space and/or time and applied at a QSH edge, can
photoexcite electron wavepackets by intra-branch electrical transitions,
without invoking the bulk states or the Zeeman coupling. Such wavepackets are
spin-polarised and propagate in opposite directions, with a density profile
that is independent of the initial equilibrium temperature and that does not
exhibit dispersion, as a result of the linearity of the spectrum and of the
chiral anomaly characterising massless Dirac electrons. We also investigate the
photoexcited energy distribution and show how, under appropriate circumstances,
minimal excitations (Levitons) are generated. Furthermore, we show that the
presence of a Rashba spin-orbit coupling can be exploited to tailor the shape
of photoexcited wavepackets. Possible experimental realizations are also
discussed.Comment: 17 pages, 3 Figure
On the Nature of Charge Transport in Quantum-Cascade Lasers
The first global quantum simulation of semiconductor-based quantum-cascade
lasers is presented. Our three-dimensional approach allows to study in a purely
microscopic way the current-voltage characteristics of state-of-the-art
unipolar nanostructures, and therefore to answer the long-standing
controversial question: is charge transport in quantum-cascade lasers mainly
coherent or incoherent? Our analysis shows that: (i) Quantum corrections to the
semiclassical scenario are minor; (ii) Inclusion of carrier-phonon and
carrier-carrier scattering gives excellent agreement with experimental results.Comment: 4 pages, 7 Postscript figures. Phys. Rev. Lett. (in press
Dispersionless propagation of electron wavepackets in single-walled carbon nanotubes
We investigate the propagation of electron wavepackets in single-walled
carbon nanotubes via a Lindblad-based density-matrix approach that enables us
to account for both dissipation and decoherence effects induced by various
phonon modes. We show that, while in semiconducting nanotubes the wavepacket
experiences the typical dispersion of conventional materials, in metallic
nanotubes its shape remains essentially unaltered, even in the presence of the
electron-phonon coupling, up to micron distances at room temperature.Comment: 4 pages, 2 figures, accepted by Appl. Phys. Let
Wigner-function formalism applied to semiconductor quantum devices: Need for nonlocal scattering models
In designing and optimizing new-generation nanomaterials and related quantum
devices, dissipation versus decoherence phenomena are often accounted for via
local scattering models, such as relaxation-time and Boltzmann-like schemes.
Here we show that the use of such local scattering approaches within the
Wigner-function formalism may lead to unphysical results, namely anomalous
suppression of intersubband relaxation, incorrect thermalization dynamics, and
violation of probability-density positivity. Furthermore, we propose a
quantum-mechanical generalization of relaxation-time and Boltzmann-like models,
resulting in nonlocal scattering superoperators that enable one to overcome
such limitations.Comment: 12 pages, 7 figure
Exciton-exciton interaction engineering in coupled GaN quantum dots
We present a fully three-dimensional study of the multiexciton optical
response of vertically coupled GaN-based quantum dots via a
direct-diagonalization approach. The proposed analysis is crucial in
understanding the fundamental properties of few-particle/exciton interactions
and, more important, may play an essential role in the design/optimization of
semiconductor-based quantum information processing schemes. In particular, we
focus on the interdot exciton-exciton coupling, key ingredient in recently
proposed all-optical quantum processors. Our analysis demonstrates that there
is a large window of realistic parameters for which both biexcitonic shift and
oscillator strength are compatible with such implementation schemes.Comment: 3 two-column pages, 3 figure
Quantum interference in nanometric devices: ballistic transport across arrays of T-shaped quantum wires
We propose that the recently realized T-shaped semiconductor quantum wires
(T-wires) could be exploited as three-terminal quantum interference devices.
T-wires are formed by intersecting two quantum wells (QWs). By use of a
scattering matrix approach and the Landauer-B\"uttiker theory, we calculate the
conductance for ballistic transport in the parent QWs and across the wire
region as a function of the injection energy. We show that different
conductance profiles can be selected by tailoring the widths of the QWs and/or
combining more wires on the scale of the Fermi wavelength. Finally, we discuss
the possibility of obtaining spin-dependent conductance of ballistic holes in
the same structures.Comment: To appear in the 09/15/97 issue of Appl. Phys. Lett. (9 pages in
REVTEX + 2 figures in postscript
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